The possible interaction between early postnatal social stimulation, derived from peers as well as the mother, and the action of testosterone in controlling the sexual differentiation of patterns of sex partner preference, sexual motivation, and coital performance will be assessed in a carnivore, the ferret. The incidence of maternal licking of the genital region of male versus female ferret kits will be studied, and the possible consequences for the neonatal secretion of testosterone in the two sexes will be assessed. These experiments should show whether early social factors can enhance testosterone secretion in males during neonatal life when this steroid is known to promote the completion of behavioral sexual differentiation in this sex. The effects of neurotoxic lesions of a sexually dimorphic male nucleus of the preoptic/anterior hypothalamic area (Mn-POA/AH) on both the development and adult expression of sex partner preference as well as motivational and performance aspects of psychosexual function will be studied in male ferrets. Also, the ability of coital stimulation to activate the expression of the proto-oncogene, c-fos, in gonaldotropin-releasing-hormone containing (GnRH) neurons as well as in non-GnRH neurons of the dorsal POA/AH and other forebrain regions will be compared in male and female ferrets using immunocytochemistry. These studies should help establish the functional significance of this sexually dimorphic structure. Finally, the ontogeny of estradiol receptors in the POA/AH and other brain regions as well as the migration of neurons along the processes of radial glial cells into the POA/AH will be compared in fetal male and female ferrets using auto-radiographic and immunocytochemical methods. The ability of prenatal steroid manipulations to alter these events will also be studied. These experiments should help establish the mechanism whereby estradiol acts during fetal development to promote the formation of the sexually dimorphic Mn-POA/AH in males. Mechanistic studies of behavioral, neuroendocrine, and neural aspects of sexual differentiation using a higher mammalian model, the ferret, should provide insight into these processes in man.